1. Field of the Invention
The present invention relates to an improved method for hydroformylation which comprises reacting an olefin with carbon monoxide and hydrogen in a hydroformylation reaction zone in the presence of a rhodium catalyst having an organophosphorus compound as a ligand. More particularly, the present invention relates to a method for hydroformylation of an olefin, which is characterized in that an unreacted olefin is recovered efficiently and economically from a reaction solution withdrawn from the hydroformylation reaction zone without substantially deactivating the rhodium catalyst, and the recovered unreacted olefin is supplied to the hydroformylation reaction zone.
2. Discussion of Background
A method has heretofore been known wherein the hydroformylation reaction of an olefin is carried out in the presence of a rhodium catalyst. Further, several methods have been disclosed for separating an unreacted olefin, an aldehyde product and the catalyst from the reaction solution of hydroformylation.
For example, in Japanese Unexamined Patent Publication No. 125103/1977, an unreacted olefin, hydrogen, carbon monoxide and an aldehyde product are stripped by a gas stripping reaction system by means of a single step complete mixing tank, and their vapor mixture is cooled and then subjected to flash separation to separate the aldehyde product from the unreacted olefin, hydrogen and carbon monoxide. It is disclosed that the major proportion of the gas stream containing the separated unreacted olefin is recycled to the reaction zone.
However, in such a case, it is apparently impossible to completely separate the olefin only by the flash separation, since the olefin has a large latent heat for distillation, and there will be a substantial loss of the olefin.
To prevent such a substantial loss of an olefin, Japanese Unexamined Patent Publication No. 101633/1991 discloses a method for recovering an olefin by means of a scrubber installation. In such a recovery method, however, the olefin and aldehydes are absorbed by a stripped catalyst solution and recycled to the reaction apparatus, whereby high boiling point products such as dimmers and trimmers formed by self condensation of aldehydes or esters are expected to increase.
Further, Japanese Unexamined Patent Publication No. 70634/1984 discloses a method for producing an aldehyde which comprises reacting an olefin with carbon monoxide and hydrogen in the presence of a water-soluble rhodium-phosphine complex compound, wherein as a method for recovering an unreacted olefin, a stripping tower by means of a synthetic gas, is employed. However, in this method, the catalyst solution is separated into a water tank in the water phase-organic phase separation step prior to introduction to the stripping tower, and therefore the catalyst solution is not substantially contained.
Still further, Japanese PCT Publication No. 502449/1991 discloses a method of intimately contacting at least a part of the stream of the hydroformylation reaction product with carbon monoxide and a hydrogen-supplying substance. However, in this method, the stream of the reaction product is withdrawn as a steam stream from the upper portion of the reactor, and therefore the catalyst is not contained in the stream of the reaction product.
On the other hand, J. Chem. Soc. (A), 1968, p. 2660 et seq, discloses that a dimmer will be formed by contacting carbon monoxide and hydrogen to Rh hydride complex under atmospheric pressure at room temperature, whereby deterioration of the activity of the Rh catalyst is expected.
Thus, a method for recovering an unreacted olefin without reducing the catalytic activity from a reaction production stream containing a catalyst solution as in a liquid-circulation process as described e.g. in U.S. Pat. No. 4,148,830, has not been known.
By conventional techniques, it has been difficult to recover an unreacted olefin by means of a countercurrent contact tower when the hydroformylation reaction is carried out in the presence of a Rh catalyst in a liquid-circulating process wherein a reaction solution withdrawn from the reaction zone contains a catalyst solution, since deterioration of the activity of the Rh catalyst will result.